1. Field of the Invention
The present invention relates to apparatus and methods for the measurement of gas flow rate. More specifically, it relates to apparatus and methods which are appropriate for obtaining standardized or otherwise normalized gas flow rate measurements.
2. Description of the Related Art
Applications for apparatus and methods useful in measuring the flow rate of a gas are numerous and widespread. The chemical processing industries would be incapable of producing many products without the ability to accurately measure gas flow rate. As technologies and technological capabilities have advanced and requirements have become ever more exacting, the demands for gas flow rate measurement apparatus and methods with increased speed and accuracy have grown.
As an example of applications with increased technical sensitivity and demands, one may consider recent attempts to further lower emissions from automobiles, trucks and the like, i.e., within the automotive industry. Under the Clean Air Act of 1990, automobile manufacturers must produce vehicles with cleaner burning engines. In order to develop engines with reduced emissions, automobile manufacturers have required a measurement apparatus and method to measure the vehicle exhaust emissions. In addition, certain certification agencies have had a requirement for a measurement apparatus and method to measure the vehicle exhaust emissions in order to certify that a particular vehicle indeed meets the legislative requirements.
The approach usually used to measure the exhaust emissions from a vehicle involves measuring the exhaust flow rate, and performing a chemical speciation analysis of the exhaust gas to identify its chemical components and their respective concentrations. This information then can be used in known manner to determine the amount of particular constituents in the exhaust emissions. The present invention in this application is quite suitable for performing the former function, i.e., quickly and accurately determining the gas flow rate for the exhaust gases, including normalizing that measured flow rate using one or more state variables for the gas, such as temperature and/or pressure.
Publicly known apparatus and methods for measuring gas flow rate typically use indirect methods of measurement. In a system publicly known as the "constant volume sampling" ("CVS") system, for example, the gas flow rate is measured by diluting the incoming gas with a diluent gas, usually ambient air. Such systems typically use either a critical flow venturi or positive displacement pump to undertake the measurement. The flow rate of the diluted mixture is then measured and calculations are performed to determine the exhaust flow rate.
Systems such as the CVS system described here have been disadvantageous, for example, in that the use of dilution air and the corresponding calculations necessitated by this approach introduce errors into the measurement which limit the accuracy and usefulness of the system and method for practical modern exhaust emissions measurement. Approaches such as that used in the CVS system also are limited in that they are relatively slow and cumbersome as an inherent consequence of their design.